Could Earth Survive Sun’s Demise?

byPaul GilsteronSeptember 14, 2007

I, for one, wouldn’t want to be around to witness what happens when the Earth is faced with an ever expanding Sun that has exhausted its hydrogen fuel. Conventional wisdom has it that the planet will likely be engulfed by what will then become a red giant. Certainly Mercury and Venus will, and the Earth’s orbit is close enough that it may meet the same fate. But it’s intriguing to learn that other outcomes are possible.

Thus news out of Iowa State that the planet known as V 391 Pegasi b has evidently survived just such an encounter with its own star. Larger than Jupiter, the distant world in the constellation Pegasus was once situated at roughly the same distance from its parent that the Earth is from the Sun. That distance has changed over time as the star lost its outer regions in the helium flash, the onset of helium fusion that is produced as hydrogen is exhausted and contraction heats the stellar core.

Image: An artist’s conception of V 391 Pegasi b as it survives the red giant expansion of its dying sun. Credit: HELAS, the European Helio- and Asteroseismology Network.

Now located at some 1.7 AU from V 391 Pegasi, the doughty planet is still there, testimony to the durability of planetary systems, and perhaps an indication that the Earth of the distant future might survive such a catastrophe. Steve Kawaler (Iowa State), a member of the research team working on this project, puts it this way:

“The exciting thing about finding a planet around this star is that it indicates that planetary systems can survive the giant phase and the helium flash of their parent star. It bodes well for the survival of our own Earth in the distant future. Before V 391 Pegasi lost its outer regions at the helium flash, the planet orbited the star at about the same distance that the Earth orbits our sun.”

Of course, what would be left on the surface of an Earth-class survivor scarcely bears contemplating. In any case, no one can say for sure whether the Earth will escape engulfment like the inner planets. Its orbit should widen as the Sun loses mass even as tidal forces drag the planet inward. We seem to be in an ambiguous zone about which too little is known to feel confidence in the outcome. Mario Livio (Space Telescope Science Institute) is quoted in this New York Timesstory: “Earth’s fate is actually the most uncertain because it is at the border line between being engulfed and surviving.”

But the new work does show that planets in orbits closer than 2 AU can survive the red giant phase. V 391 Pegasi’s maximum radius is thought to have reached 0.7 AU, a close brush indeed with the planet in question. Earth’s fate won’t be decided for five billion years. The paper is Silvotti et al., “A giant planet orbiting the ‘extreme horizontal branch’ star V 391 Pegasi,” Nature 449 (13 September 2007), pp. 189-191 (abstract).

Not really what I’d call survival since a planet that close to a AGB phase red giant would be mostly molten rock on the sunside.

As for mass-loss much there’s two phases – during the Red Giant Branch (RGB), then the Asymptotic Giant Branch (AGB) – which lose about 25 and 20% respectively via a powerful solar wind, essentially akin to what throws off planetary nebulae.

The Helium Flash itself is quite an event, but within the star – basically the core explodes as helium-burning begins, heaving the core onto the Helium Main Sequence. That heaving against the immense gravity of the core uses up most of the energy of the explosion, thus not a lot gets through to the surface.

Thinking of tide-locking of the Earth to face the Sun, there was an article on what the Earth would be like in such a situation a few years ago (6 dec 2003) in “New Scientist”. Jeffrey Kargel predicted a 2200 C magma ocean on the sunside and a giant cap of frozen gas on the darkside, perhaps ringed by an annulus of liquid water.

Would be a very strange place as the evaporating magma would rain out oxides and metals at different temperatures thus ringing the magma with refractory aluminium & calcium oxides, then iron and silicon oxides and finally light metals like potassium and sodium just past the terminator, releasing enough heat to melt the edges of the ice-cap and make that annular ocean forever in the fringes of twilight. At least until the AGB phase ends.

As I understand it from reading a few papers about it, sdB stars (like V391 Peg) are produced from stars which lost their hydrogen envelopes on the red giant branch. sdO stars result from envelope loss on the asymptotic giant branch.

So V391 Peg’s planet may have got lucky because the evolution of the star got interrupted before it ever got to the asymptotic giant branch.

Then again, extreme horizontal branch stars (a.k.a. hot subdwarfs) are not well understood: one prediction is that they result from interactions between binary stars, but V391 Peg is apparently not a binary star. But remember we only have a minimum mass for the planet candidate, if the orbit is being observed nearly face-on, it could well be more massive (if it is a low luminosity degenerate star that would explain why the light from such a star isn’t observed), which would help explain the system’s evolution.

To destroy a terrestrial planet, you need the Death Star. But what will you do if you want to take out a gas giant? No mere superlaser is going to get the job done. But if you can get the gas giant close enough to its parent star, you should just be able to make it evaporate. How close? According to researchers from University College London, get a planet twice as close as Mercury to its parent star and it’s a goner (in a few billion years).

But whoa you say, haven’t astronomers found planets orbiting well within this distance? They certainly have. In fact, HD 209458b is 70% the mass of Jupiter and orbits its parent star about 12% the orbital distance of Mercury. And it’s evaporating as we speak.

Okay fine, it doesn’t destroy a planet in such a spectacular fashion as blasting it with a superlaser, but you can rest assured, its fate is sealed. Queue the maniacal laughter…

The research was carried out by Tommi Koskinen from University College London, and published in this week’s edition of the journal Nature.

Abstract: We revisit the distant future of the Sun and the solar system, based on stellar models computed with a thoroughly tested evolution code. For the solar giant stages, mass-loss by the cool (but not dust-driven) wind is considered in detail. Using the new and well-calibrated mass-loss formula of Schroder & Cuntz (2005, 2007), we find that the mass lost by the Sun as an RGB giant (0.332 M_Sun, 7.59 Gy from now) potentially gives planet Earth a significant orbital expansion, inversely proportional to the remaining solar mass.

According to these solar evolution models, the closest encounter of planet Earth with the solar cool giant photosphere will occur during the tip-RGB phase. During this critical episode, for each time-step of the evolution model, we consider the loss of orbital angular momentum suffered by planet Earth from tidal interaction with the giant Sun, as well as dynamical drag in the lower chromosphere. We find that planet Earth will not be able to escape engulfment, despite the positive effect of solar mass-loss. In order to survive the solar tip-RGB phase, any hypothetical planet would require a present-day minimum orbital radius of about 1.15 AU.

Furthermore, our solar evolution models with detailed mass-loss description predict that the resulting tip-AGB giant will not reach its tip-RGB size. The main reason is the more significant amount of mass lost already in the RGB phase of the Sun. Hence, the tip-AGB luminosity will come short of driving a final, dust-driven superwind, and there will be no regular solar planetary nebula (PN). But a last thermal pulse may produce a circumstellar (CS) shell similar to, but rather smaller than, that of the peculiar PN IC 2149 with an estimated total CS shell mass of just a few hundredths of a solar mass.

Paris (AFP) Feb 21, 2008 – The big news: Earth is doomed
to fry and then be gobbled up by the dying Sun. But don’t
blow your savings on an Apocalypse Party just yet, for
astronomers say the planet’s demise is 7.6 billion years
away.

The unusual calculations appear in the British open-access
journal Astrophysics. Robert Smith, emeritus reader in
astronomy at the University of Sussex … more

Nuclear reactors could be burning
deep beneath the ground, two
scientists have claimed. They say
that uranium could become
sufficiently concentrated at the
base of Earth’s mantle to ignite
self-sustained nuclear fission, as
in a human-made…

Classical and relativistic orbital effects of Sun’s mass loss and the Earth’s fate

Authors: Lorenzo Iorio

(Submitted on 18 Jun 2008)

Abstract: We calculate the classical and general relativistic effects induced by an isotropic mass loss of a body on the orbital motion of a test particle around it. Concerning the Newtonian case, we perturbatively obtain negative secular rates for the osculating semimajor axis a, the eccentricity e and the mean anomaly, while the argument of pericenter $\omega$ does not undergo secular precession. Moreover, the anomalistic period is different from the Keplerian one and is larger than it. The true orbit, instead, expands, as shown by a numerical integration of the equations of motion in Cartesian coordinates. It is shown that, in fact, this is in agreement with the decreasing of a and $e$ because they refer to the osculating Keplerian ellipses which approximate the trajectory at each instant.

By assuming for the Sun \dot M/M = -9 X 10^-14 yr^-1 it turns out that the Earth’s perihelion position is displaced outward by 3 mm along the fixed line of apsides after each revolution. By applying our results to the phase in which the radius of the Sun, already moved to the Red Giant Branch of the Hertzsprung-Russell Diagram, will become as large as 1.20 AU in 1 Myr, we find that the Earth’s perihelion position on the fixed line of the apsides will increase by only 10^-2 AU (for \dot M/M = -2 X 10^-7 yr^-1).

Thus, even without invoking tidal effects and drag, the Earth should not avoid the engulfment in the expanded solar photosphere. The effects induced by general relativity consist of secular positive rates of the semimajor axis and the eccentricity. They are completely negligible in the present and future evolution of the Solar System.

The search for a strategy for mankind to survive the solar Red Giant catastrophe

Authors: M. Taube, W. Seifritz

(Submitted on 25 Nov 2008)

Abstract: In around 5 gigayears our Sun will grow to a Red Giant and will swallow Earth. The plan is subdivided into two parts: We propose to construct some kind of parasol to shadow Earth. The position of the parasol will be the (inner) Lagrange Point L1.

If we want to survive also the time beyond the next 5 Gy, where Suns luminosity and radius increase hundred fold and oscillate until our Sun develops finally into a White Dwarf, we have to shift Earth into the Kuiper Belt (50 AU) by means of the swing-by technique During this journey of about some megayears or more Earth must be illuminated by an artificial light source.

A ring of DD-fusion power stations outstretched on Moons orbit should produce the necessary 175 PW of visible light. In the Kuiper Belt Earth will be brought into an orbit of an artificial Sun, an ArtSun formed in the meantime by the fusion of gaseous Jupiter-like planets imported from other planetary systems in the neighborhood

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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